1. The Most Secure Cloud Storage Provider
Anton Titov CTO
CyberCon, June 2018, Sofia

2. Client-side encryption Zero-knowledge privacy Multi-layer protection
Absolute Security with pCloud Crypto
Client-side encryption
Only the user has access to the encrypted content
Zero-knowledge privacy
pCloud does not know what is saved in Crypto Folders
Multi-layer protection
Additional protection layers & keys
pCloud Crypto Hacking Challenge with $100,000 award.
For 6 months
2,800+ hackers - Berkeley, Boston, MIT and 523 other organizations
None of them succeeded.

3. Mission Client-side encrypted cloud storage
Accessible from multiple software clients / platforms
All user needs to access encrypted data are account credentials and encryption passphrase
Efficient read/write random access

4. Everybody do encryption, don't they?
Encryption of in-transit data
Encryption of stored data with provider's keys
Client-side encryption

5. Still, it should not be that hard, right?
What is actually encryption?
What is authentication?
Encryption and authentication of data connection - fairly trivial.
Block device (hard drive) encryption - fairly trivial. Authentication may degrade performance, but you can live without it.
Encryption of randomly accessible data - fairly trivial, authentication is however tricky.

6. How to authenticate a file as a whole?
Split file into blocks of some size, say 4Kb
Use a Merkle tree to authenticate the file
Since we are working with disk storage, don't use a binary Merkle tree, but say 256-way one
Files with reasonable size will have a tree-depth of 2-4 levels
Wouldn't it be great to also have a Merkle tree of directory structure?

7. Technological overview
Each user has 4096 bit RSA key, private encrypted with AES256 key derived from passphrase with PBKDF2, public and encrypted private parts stored in the cloud
Each folder and each file has its own AES256 and HMAC-SHA512 key, encrypted with RSA and stored in the cloud
Folder keys used to encrypt file names and sign keys of in-folder contents
File keys encrypt and authenticate the file data

8. So the private key is actually stored in the cloud?
Yes, but it is in turn encrypted
There are alternatives but those are slow and messy

9. Future work: Share-Able encryption
24/7
Access to files
We don't really need RSA in the picture now
Big challenge: trust, MITM
Just encrypt every folder/file key with public keys of all users that have access
Linear scaling by number of users is not ideal, but public RSA operations are ~100 faster than private ones anyway
Collaboration

10. Anton Titov, CTO anton@pCloud.com
Thank You
Questions?
Anton Titov, CTO